This invention relates to gas-lubricated bearings and to methods of forming such bearings.
The invention is more especially concerned with hydrodynamic gas-lubricated bearings, that is, a bearing in which a lubricating gas film is generated by the relative movement between the bearing surfaces, rather than by an external source of pressurized gas.
Hydrodynamic gas-lubricated bearings comprise two surfaces one of which is smooth and the other of which is provided with a pattern of shallow grooves. In a rotary thrust bearing, the surfaces may, for example, be formed on flat disc-shape bearing members and, in this example, the grooves may be of a logarithmic spiral configuration. Relative rotational movement between the two surfaces causes gas to be entrained into the grooves and between the two surfaces, building up a film that separates the two bearing members, thereby permitting low-friction rotation. Bearings of this form are well-known and are described, for example, in U.K. Patent Specification Nos. 1 310 523, 1 310 524, 1 310 525 and 1 310 526. It is important, in the manufacture of such bearings that the surfaces have a polished and optically-flat finish and that the depth and profile of the grooves be within narrowly-defined limits. Ways of manufacturing such bearing surfaces are described in U.K. Patent Specification No. 1 258 741, 1 298 456 and 1 299 452.
The bearing surface need not be flat but can instead be on hemispherical or cylindrical surfaces.
When a gas-lubricated bearing is at rest the two bearing surfaces are normally in contact with one another and, because of the accurately formed surfaces, the area of contact can be relatively large. Problems can arise when starting such bearings before the gas film has fully separated the bearing surfaces, since the friction between the contacting regions can cause abrasion, wear and frictional heating. This can be an especially severe problem when the load on the bearing surfaces is high and in some cases can cause "wringing" between the two surfaces, that is, adhesion between the two surfaces caused by the intimate contact between the surfaces and the expelling of gas from between them.
In an attempt to overcome this problem with flat bearing surfaces it has been proposed to crown one or the other of the surfaces, as shown in FIG. 1 (to an exaggerated extent) by lapping it against a slightly concave lapping plate so as thereby to make the bearing surface slightly convex relative to the other surface. In this manner, the two bearing surfaces will contact only at the centre of the bearing surfaces thereby reducing the friction radius arm and the area of contact. It will be appreciated that the curvature on the crowned surface is only very slight and it has been found that it is difficult and expensive to form this surface accurately by lapping without a significant loss in the performance of the bearing.